Abstract : The results of a systematic and statistical study of clear air turbulence encountered along the main airline routes over the Soviet Union and the socialist countries of Eastern Europe are now available in summary form. The data were collected by crews of turboprop and turbojet aircraft, generally flying at altitudes of 6-10.5 km, for four 5-day periods (9-13 December 1964, 10-14 March, 9-13 June, and 8-12 September 1965). The data were grouped into four 6-hour daily observation periods (2400-0559, 0600-1159, 1200-1759, and 1800-2359 hr Greenwich time). Some of the results of a similar study made by Colson in 1962 for the United States, and those obtained in an earlier study by Soviet scientists, are compared with the data of the present study. Major aspects of the analysis include determination of (1) the probability of encountering turbulent zones in various regions of and altitudes above the area covered by the study, (2) the relationship of turbulence to wind speeds and directions, vertical wind shear, Richardson number, and horizontal wind shear. Data on actual encounters with turbulence are presented for the four 5-day periods in tables and are graphically portrayed on four small-scale maps.